During the past year we have assessed the effects of repeated doses of intravenous cocaine on selected physiological, behavioral and biochemical measures in order to continue to explore the possibilities of either tolerance or drug sensitization. In addition, we have begun a series of studies utilizing a choice paradigm to establish a hierarchy of cocaine preference by route and dose. Our aim is to eventually be able to elucidate some of the variables which can be manipulated to modify the reinforcing actions of cocaine. We plan to continue this research by giving volunteer participants, living within our controlled hospital environment, the opportunity to work for drug or points which can be exchanged for various available commodities, including cocaine or money. The data derived from these studies should enable us to characterize the environmental, behavioral and pharmacological variables which contribute to, or can be effective in, modifying human drug abuse.